Recently, non-volatile memory devices, such as a NAND type flash memory, are used. Such a memory device may take a considerably longer time for data writing than a time needed for data reading. This is because an erasure operation is possible only in a large data size of a block, so that even writing data with a smaller data size requires three processes (a) reading from the entire block, (b) changing read-out data, and (c) writing in the entire block.    [Patent Document 1]    Japanese Unexamined Patent Publication (Kokai) No. 2004-303232